Wireless communications have become a major way of communications and in many applications are replacing conventional land-based communication systems. There are many kinds of wireless communication systems, such as the cellular phone system, the wireless LAN, and the WiMAX. The most commonly used system is the cellular system, and the other wireless communication systems such as WiFi and WiMax are growing rapidly.
The users of a wireless channel may be, for instance, a cellular phone, a laptop computer, etc. Many users may share same communication channel. Therefore, they interfere with each other. A typical communication objective is for the receiver to receive the transmitting signal from a specific user with whom the receiver wants to communicate. In order to achieve this objective, multiple access systems are used. Typical multiple access systems are the Frequency Division Multiple Access technology (FDMA), the Time Division Multiple Access technology (TDMA), and the Code Division Multiple Access technology (CDMA). Each multiple access system establishes its own rules for their users and uses base stations to regulate their users to achieve this objective.
One common problem frequently encountered in wireless communications is the presence of interfering signals transmitted by devices other than the particular sender with which the receiver wants to communicate. Depending on the types of the wireless communications, interferences can be classified in many ways. It may be intentional, such as the jamming of military wireless transmissions. It may be accidental and resulting from having multiple users who are sharing a common wireless channel with or without the same base station that regulates the particular user. The interferences resulting from users with the same base station are called the multiple access interferences (MAI). All others are called the co-channel interferences. For simplicity, both interferences are lumped into one word interferences.
The presence of interferences can severely compromise the ability of the receiver to discern the signal from the intended sender, resulting in a significant reduction of the channel data rate for the wireless transmissions from the sender to the receiver. In the cellular system, multiple access systems have been used to combat the multiple access interference with the support of a base station. The three commonly used multiple access technologies are the FDMA, the TDMA, and the CDMA. In FDMA, the users are assigned non-overlapping frequency slots (by the base station), and hence the multiple access interference can be avoided. Similarly, in TDMA the users are assigned non-overlapping time-slots, and in CDMA users are assigned ‘non-overlapping’ orthogonal codes. Because of the power limitation of the base station, the area of the cell it controls is limited to its neighborhood. Therefore, any user outside the cell can not be controlled by this base station, and hence may interfere with the users in the cell.
Interferences in wireless transmissions are also a serious issue for Wireless Local Area Network (LAN). A wireless LAN uses part of the frequency spectrum that is free to everybody and hence it costs nothing to use the spectrum. It uses the internet to reach the outside world and hence it again costs nothing. However, the interferences between the users can be significant because this part of spectrum is unregulated. This is one of the major challenges for wireless LAN. The current technology confines it to be in a local area with limited users. It does not allow it to be developed to reach its full commercial potential value. WiFi is one kind of Wireless LAN and hence shares the same advantages and challenges. Interferences between the users also exist in ad hoc network, a closed wireless communication network but without a based station for central command.
The number of users that is allowed within a fixed spectrum is also limited by the interference. Each company is assigned a total spectrum for its exclusive use. According to its multiple access system, the total spectrum is then divided into a number of “personal spectrums”. These personnel spectrums are preferred to be non-overlapping so that there are no interferences between users. This limits the number of users that is allowed within a fixed spectrum. However, higher data rate would demand a wider personal spectrum for the users. This could cause serious interferences between the users because of the heavy overlapping of their personal spectrums. Therefore, an effective way of blocking these interferences would have the potential of increasing the number of users within a fixed spectrum.
In view of the foregoing, there is a vital need for a way to effectively counter the negative effects of interfering signals and to enhance the channel data rate of wireless communications between a sender and a receiver in the presence of interfering signals, and to increase the number of users within a fixed spectrum.
With the thirst of ever increasing higher data rate by demand, the current multiuser wireless communications systems have difficult to fulfill the challenge for future generations of multiple access systems. For example, the required data rate for digital voice is less than 64 Kbps, Standard Digital TV (SDTV) is 3-6 Mbps, and High Definition TV (HDTV) is 18-24 Mbps. On the other hand, the data rate of 2 G cellular system is about 140 kbps that is enough to support digital voice and text transmission. The 3 G standards call for data rates up to 2 Mbps which can support the transfer of SDTV. Future wireless data applications such as broadband Internet access, interactive 3D gaming, and high quality audio and video entertainment, each of them may require a date rate of 1-5 Mbps, which is much higher than current cellular phone system can provide.
The invention of the specific MIMO multiple access system described in this patent application can reduce substantially the interferences at the receiver and hence increases significantly the channel data rate, without the increase of spectrum bandwidth. A positive consequence of this is the increase of number of users within a fixed spectrum.